Illumination system comprising a radiation source and a fluorescent material

ABSTRACT

The invention concerns an illumination system for generation of colored, especially amber or red light, comprising a radiation source and a fluorescent material comprising at least one phosphor capable of absorbing a part of light emitted by the radiation source and emitting light of wavelength different from that of the absorbed light; wherein said at least one phosphor is a amber to red emitting a rare earth metal-activated oxonitridoalumosilicate of general formula (Ca 1-x-y-z Sr x Ba y Mg z ) 1-n (Al 1-a+b B a )Si 1-b N 3-b O b :Re n , wherein 0≦x≦1, 0≦y≦1, 0≦z≦1, 0≦a≦1, 0&lt;b≦1 and 0.002≦n≦0.2 and RE is selected from europium(II) and cerium(III).

BACKGROUND

1. Field of Invention

The present invention generally relates to an illumination systemcomprising a radiation source and a fluorescent material comprising anamber to red phosphor. The invention also relates to an amber to redphosphor for use in an illumination system. More particularly, theinvention relates to an illumination system comprising anelectroluminescent semiconductor device as radiation source and afluorescent material comprising a phosphor for the generation ofspecific, white or colored light, including yellow, amber and red light.In this illumination system white or colored light is generated byluminescent down conversion and additive color mixing based on anultraviolet or blue primary radiation. A solid-state light-emittingdiode as a source of primary radiation is especially contemplated.

2. Description of Related Art

Various illumination systems for vehicular and signaling usage areknown. Vehicles include a number of different components and assembliesthat have an illuminator or a signal lamp associated therewith. Greatinterest has been shown in the use of electroluminescent semiconductordevices, such as solid-state light emitting diodes (LEDs), asilluminators and signal indicators because they offer many potentialadvantages as compared to other conventional low voltage light sources.Other light sources suffer from deficiencies, including relativeinefficiency, such as is the case with conventional tungstenincandescent lamps; high operating voltages, such as is the case withfluorescent and gas discharge lamps; or susceptibility to damage, suchas is the case with incandescent lamps.

Accordingly, these alternate light sources are not optimal for vehicularapplications where only limited power or low voltage is available, orwhere high voltage is unacceptable for safety reasons, or inapplications where there is significant shock or vibration. LEDs on theother hand are highly shock resistant, and therefore provide significantadvantages over incandescent and fluorescent bulbs, which can shatterwhen subjected to mechanical or thermal shock. LEDs also possessoperating lifetimes from 200,000 hours to 1,000,000 hours, as comparedto the typical 1,000 to 2,000 hours for incandescent lamps or 5,000 to10,000 hours for fluorescent lamps.

Current yellow, amber or red traffic and vehicular lights comprising anelectroluminescent semiconductor device, rely on direct excitation ofaluminum gallium indium phosphide (AlGaInP) LED chips for generation ofcolored yellow, amber or red light.

A drawback of AlInGaP LEDs is the quenching of the light emission withincreasing temperature. Their light output drops by more than 40% if thetemperature is raised from room temperature to 100° C. At the same timethe spectrum shifts, e.g. from 617 nm to 623 nm, which reduces theluminous efficacy further. Therefore, there is a strong demand by theautomotive industry for yellow to red LEDs with a much smallerdependence of the light yield and emission spectrum on temperature.

One presently discussed solution for generation of yellow, amber or redlight is the application of white LEDs and an appropriate colour filter,since the AlInGaN chips applied in white LEDs show much less thermalquenching. In addition, the spectral shift of white LEDs withtemperature is less severe due to the application of the YAG:Cephosphor. However, the major drawback of this concept is the lowefficiency due to the fact that the present white LEDs emit only a fewpercent orange to red light and most of the white LED spectrum is cutoff.

Another approach is known, e.g. from U.S. Pat. No. 6,649,946 wherein alight source for generating of yellow to red light by using ayellow-to-red-emitting phosphor is disclosed. Said phosphor has a hostlattice of the nitridosilicate type M_(x)Si_(y)N_(z):Eu, wherein M is atleast one of an alkaline earth metal chosen from the group Ca, Sr, Ba,Zn and wherein z=⅔x+ 4/3y. The phosphor can be used to create a highlystable red or orange or yellow emitting LED which may be based on aprimary light source (preferably InGaN-chip) of peak emission around 380to 480 nm whose light is fully converted by a nitride phosphor of theinventive type rare-earth activated silicon nitrides doped with Eu.These LEDs show higher efficiency and improved stability compared towell-known commercial LEDs with direct excitation of yellow to redcolors.

Yet, a recent evaluation of the chromaticity requirements for trafficsigns has indicated that the red color range of vehicular and trafficsigns should include a longer-wavelength cut-off to ensure detection ofthe signal by color vision deficient drivers.

Therefore, there is a need to provide an illumination system comprisingphosphors that are excitable by a radiation source of the nearUV-to-blue range and emit in the visible amber to deep-red range.

SUMMARY

Thus the present invention provides an illumination system, comprising aradiation source and a fluorescent material comprising at least onephosphor capable of absorbing a part of light emitted by the radiationsource and emitting light of wavelength different from that of theabsorbed light; wherein said at least one phosphor is a rare earthmetal-activated oxonitridoalumosilicate of general formula(Ca_(1-x-y-z)Sr_(x)Ba_(y)Mg_(z))_(1-n)(Al_(1-a+b)B_(a))Si_(1-b)N_(3-b)O_(b):Re_(n),wherein 0≦x≦1, 0≦y≦1, 0≦z≦1, 0≦a≦1, 0<b≦1 and 0.002≦n≦0.2 and RE isselected from europium(II) and cerium(III).

An illumination system according to the present invention can provide acomposite colored output light, especially amber or red light with ahigh temperature resistance, color point stability and a high efficiencyat the same time.

In particular, the composite output light has a greater amount ofemission in the deep red color range than the conventional lamp andenlarges the range of colors that can be reproduced. This characteristicmakes the device ideal for applications, such as yellow, amber and redtraffic lighting, stair/exit ramp lighting, decorative lighting andsignal lighting for vehicles.

An illumination system according to the present invention can alsoprovide a composite white output light that is well balanced withrespect to color. In particular, the composite white output light has agreater amount of emission in the red color range than the conventionallamp. This characteristic makes the device ideal for applications inwhich a true color rendition is required. Such applications of theinvention include, inter alia, traffic lighting, street lighting,security lighting and lighting of automated factories.

Especially contemplated as the radiation source is a solid-state lightemitting diode. According to a first aspect of the invention a lightillumination system comprises a blue-light emitting diode having a peakemission wavelength in the range of 420 to 495 nm as a radiation sourceand a fluorescent material comprising at least one phosphor, that is arare earth metal-activated oxonitridoalumosilicate of general formula(Ca_(1-x-y-z)Sr_(x)Ba_(y)Mg_(z))_(1-n)(Al_(1-a+b)B_(a))Si_(1-b)N_(3-b)O_(b):RE_(n),wherein 0≦x≦1, 0≦y≦1, 0≦z≦1, 0≦a≦1, 0<b≦1 and 0.002≦n≦0.2 and RE isselected from europium(II) and cerium(III).

Such illumination system will provide white or colored, especially amberor red light in operation. The blue light emitted by the LED excites thephosphor, causing it to emit amber or red light. The blue light emittedby the LED is transmitted through the phosphor and is mixed with theamber or red light emitted by the phosphor. The viewer perceives themixture of blue and amber light as white or colored light, depending onthe amount of phosphor present in the fluorescent material.

According to one embodiment the invention provides a white or colored,especially amber or red light illumination system comprising ablue-light emitting diode having a peak emission wavelength in the rangeof 420 to 495 nm as a radiation source and a fluorescent materialcomprising a rare earth metal-activated oxonitridoalumosilicate ofgeneral formula(Ca_(1-x-y-z)Sr_(x)Ba_(y)Mg_(z))_(1-n)(Al_(1-a+b)B_(a))Si_(1-b)N_(3-b)O_(b):RE_(n),wherein 0≦x≦1, 0≦y≦1, 0≦z≦1, 0≦a≦1, 0<b≦1 and 0.002≦n≦0.2 and RE isselected from europium(II) and cerium(III) and at least one secondphosphor.

When the fluorescent material comprises a phosphor blend of a phosphorof the rare earth metal-activated oxonitridoalumosilicate type and atleast one second phosphor the color rendition of the white or coloredlight illumination system according to the invention may be furtherimproved.

In particular, the fluorescent material may be a phosphor blend,comprising a rare earth metal-activated oxonitridoalumosilicate ofgeneral formula(Ca_(1-x-y-z)Sr_(x)Ba_(y)Mg_(z))_(1-n)(Al_(1-a+b)B_(a))Si_(1-b)N_(3-b)O_(b):RE_(n),wherein 0≦x≦1, 0≦y≦1, 0≦z≦1, 0≦a≦1, 0<b≦1 and 0.002≦n≦0.2 and RE isselected from europium(II) and cerium(III) and a red phosphor. Such redphosphor may be selected from the group of Ca_(1-x-y)Sr_(x)S:Eu_(y),wherein 0≦x≦1 and 0<y≦0.2; and (Sr_(1-x-y)Ba_(x)Ca_(y))₂Si₅N₈:Eu_(z)wherein 0≦x≦1, 0≦y≦1 and 0<z≦0.2.

Otherwise the fluorescent material may be a phosphor blend, comprising arare earth metal-activated oxonitridoalumosilicate of general formula(Ca_(1-x-y-z)Sr_(x)Ba_(y)Mg_(z))_(1-n)(Al_(1-a+b)B_(a))Si_(1-b)N_(3-b)O_(b):RE_(n),wherein 0≦x≦1, 0≦y≦1, 0≦z≦1, 0≦a≦1, 0<b≦1 and 0.002≦n≦0.2 and RE isselected from europium(II) and cerium(III) and a yellow-to-greenphosphor. Such yellow-to-green phosphor may be selected from the groupcomprising (Ca_(x)Sr_(1-x-y))₂SiO₄:Eu_(y), wherein 0≦x≦ and 0<y≦0.2,(Sr_(x)Ba_(1-x-y))₂SiO₄:Eu_(y), wherein 0≦x≦1 and 0<y≦0.2,(Sr_(1-x-y)Ba_(x))Ga₂S₄:Eu_(y) wherein 0≦x≦1 and 0<y≦0.2,(Y_(1-x-y-z)Gd_(x)Lu_(z))₃(Al_(1-a)Ga_(a))₅O₁₂:Ce_(y), wherein 0≦x≦1 and0<y≦0.2, 0≦z≦1, 0≦a≦0.5, ZnS:Cu, CaS:Ce,Cl, and SrSi₂N₂O₂:Eu.

The emission spectrum of such a fluorescent material comprisingadditional phosphors has the appropriate wavelengths to obtain togetherwith the blue light of the LED and the amber to red light of the rareearth metal-activated oxonitridoalumosilicate type phosphor according tothe invention a high quality colored light with good color rendering atthe required color temperature.

According to another aspect of the invention there is provided a whiteor colored especially amber or red light illumination system, whereinthe radiation source is selected from the light emitting diodes havingan emission with a peak emission wavelength in the UV-range of 200 to400 nm and the fluorescent material comprises at least one phosphor,that is a rare earth metal-activated oxonitridoalumosilicate of generalformula(Ca_(1-x-y-z)Sr_(x)Ba_(y)Mg_(z))_(1-n)(Al_(1-a+b)B_(a))Si_(1-b)N_(3-b)O_(b):RE_(n),wherein 0≦x≦1, 0≦y≦1, 0≦z≦1, 0≦a≦1, 0<b≦1 and 0.002≦n≦0.2 and RE isselected from europium(II) and cerium(III).

According to one embodiment the invention provides a white lightillumination system comprising a blue-light emitting diode having a peakemission wavelength in the UV-range of 200 to 420 nm as a radiationsource and a fluorescent material comprising a rare earthmetal-activated oxonitridoalumosilicate of general formula(Ca_(1-x-y-z)Sr_(x)Ba_(y)Mg_(z))_(1-n)(Al_(1-a+b)B_(a))Si_(1-b)N_(3-b)O_(b):RE_(n),wherein 0≦x≦1, 0≦y≦1, 0≦z≦1, 0≦a≦1, 0<b≦1 and 0.002≦n≦0.2 and RE isselected from europium(II) and cerium(III) and at least one secondphosphor.

In particular, the fluorescent material may be a phosphor blend,comprising a rare earth metal-activated oxonitridoalumosilicate ofgeneral formula(Ca_(1-x-y-z)Sr_(x)Ba_(y)Mg_(z))_(1-n)(Al_(1-a+b)B_(a))Si_(1-b)N_(3-b)O_(b):RE_(n),wherein 0≦x≦1, 0≦y≦1, 0≦z≦1, 0≦a≦1, 0<b≦1 and 0.002≦n≦0.2 and RE isselected from europium(II) and cerium(III) and a blue phosphor. Suchblue phosphor may be selected from the group comprisingBaMgAl_(1o)O₁₇:Eu, Ba₅SiO₄(Cl,Br)₆:Eu, CaLa₂S₄:Ce,(Sr,Ba,Ca)₅(PO₄)₃Cl:Eu and LaSi₃N₅:Ce.

The emission spectrum of such a fluorescent material comprisingadditional phosphors has the appropriate wavelengths to obtain togetherwith the blue light of the LED and the amber to red light of the rareearth metal-activated oxonitridoalumosilicate type phosphor according tothe invention a high quality colored light with good color rendering atthe required color temperature.

Another aspect of the present invention provides a phosphor capable ofabsorbing a part of light emitted by the radiation source and emittinglight of wavelength different from that of the absorbed light; whereinsaid phosphor is a rare earth metal-activated oxonitridoalumosilicate ofgeneral formula(Ca_(1-x-y-z)Sr_(x)Ba_(y)Mg_(z))_(1-n)(Al_(1-a+b)B_(a))Si_(1-b)N_(3-b)O_(b):RE_(n),wherein 0≦x≦1, 0≦y≦1, 0≦z≦1, 0≦a≦1, 0<b≦1 and 0.002≦n≦0.2 and RE isselected from europium(II) and cerium(III).

By varying the chemical composition of the phosphor, the phosphor colorcan be shifted from amber to a deep red. The emission spectra extend inrelatively inaccessible regions of the spectrum, including the deep redand near infrared.

The fluorescent material is excitable by UV-A emission lines which havesuch wavelengths as from 300 nm to 400 nm, but is excited also with highefficiency by blue light emitted by a blue light emitting diode having awavelength around 400 to 495 nm. Thus the fluorescent material has idealcharacteristics for conversion of blue light of nitride semiconductorlight emitting component into white or colored light. These phosphorsare broadband emitters wherein the visible emission is so broad thatthere is no 80 nm wavelength range where the visible emission ispredominantly located. Total conversion efficiency can be up to 98%.

Additional important characteristics of the rare earth metal-activatedoxonitridoalumosilicate type phosphors include 1) resistance to thermalquenching of 30 luminescence at typical device operating temperatures(e.g. 80° C.); 2) lack of interfering reactivity with the encapsulatingresins used in the device fabrication; 3) suitable absorptive profilesto minimize dead absorption within the visible spectrum; 4) a temporallystable luminous output over the operating lifetime of the device and; 5)compositionally controlled tuning of the phosphors excitation andemission properties. These rare earth metal-activatedoxonitridoalumosilicate type phosphors may also include ytterbium,praseodymium, samarium and other cations including mixtures of cationsas co-activators.

The phosphors may have a coating selected from the group of fluoridesand orthophosphates of the elements aluminum, scandium, yttrium,lanthanum gadolinium and lutetium, the oxides of aluminum, yttrium,silicon and lanthanum and the nitride of aluminum.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic view of a dichromatic white LED lamp comprisinga phosphor of the present invention positioned in a pathway of lightemitted by an LED structure.

FIG. 2 shows a portion of the crystal structure of(Ca,RE)Al_(1-x+y)B_(x)Si_(1-y)N_(3-y)O_(y) in a schematic view.

FIG. 3 shows the C.I.E. chromaticity diagram with the color points offluorescent materials according to the invention and the color points ofthe fluorescent materials.

FIG. 4 shows the excitation, emission and reflection spectra of(Ca_(0.95)Sr_(0.05))_(0.98)Al_(1.04)Si_(0.96)N_(2.96)O_(0.04):Eu_(0.02).

FIG. 5 shows a cross sectional view of an alternative embodiment of anillumination system including an LED and a phosphor positioned in thepathway of light emitted by the LED.

DETAILED DESCRIPTION The Rare Earth Metal-ActivatedOxonitridoalumosilicate Phosphor

The present invention focuses on a rare earth metal-activatedoxonitridoalumosilicate as a phosphor in any configuration of anillumination system containing a radiation source, including, but notlimited to discharge lamps, fluorescent lamps, LEDs, LDs and X-raytubes. As used herein, the term “radiation” encompasses radiation in theUV, IR and visible regions of the electromagnetic spectrum.

While the use of the present phosphor is contemplated for a wide arrayof illumination, the present invention is described with particularreference to and finds particular application to phosphor-convertedlight emitting diodes comprising especially UV- and blue-light-emittingdiodes as radiation sources.

The phosphor conforms to the general formula(Ca_(1-x-y-z)Sr_(x)Ba_(y)Mg_(z))_(1-n)(Al_(1-a+b)B_(a))Si_(1-b)N_(3-b)O_(b):RE_(n),wherein 0≦x≦1, 0≦y≦1, 0≦z≦1, 0≦a≦1, 0<b≦1 and 0.002≦n≦0.2 and RE isselected from europium(II) and cerium(III).

This class of phosphor material is based on rare earth metal-activatedluminescence of an aluminum-substituted oxonitridosilicate. The phosphorcomprises a host lattice, wherein the main components are silicon,aluminum, nitrogen and oxygen. The host lattice may also comprise boron.The host layer has a structure comprising stacks of layers of[(Al,SO)₂N_(6/3)(N,O)_(2/2)], wherein silicon and aluminum aretetrahedrically surrounded by oxygen and nitrogen, as shown in FIG. 2.

The N atoms that connect two silicon or aluminum atoms can besubstituted by oxygen if the aluminum/silicon ratio is alteredsimultaneously to maintain charge balance and solid solutions within thesystem MAlSiN₃-MAl₂N₂O. Thus, oxygen defects that are incorporated inthe phosphor material by the precursor materials or the firingatmosphere can be compensated by changing the Al/Si ratio in theprecursor mix. A lower defect density in the fluorescent material leadsto higher efficiency and better photochemical stability. Within thelayers the metal cations such as earth alkaline metals as well aseuropium(II), cerium(III) and eventually a co-activator areincorporated. They are mainly coordinated by the N atoms that connecttwo aluminum or silicon atoms or by the substitutional oxygen atoms.Preferably the earth alkaline metals are selected from magnesium,calcium, strontium and barium.

The proportion z of rare earth metal is preferably in a range of0.002≦n≦0.2. When the proportion n is lower, luminance decreases becausethe number of excited emission centers of photoluminescence due to rareearth metal-cations decreases and, when the n is greater than 0.2,concentration quenching occurs. Concentration quenching refers to thedecrease in emission intensity which occurs when the concentration of anactivation agent added to increase the luminance of the fluorescentmaterial is increased beyond an optimum level.

The method for producing a microcrystalline phosphor powder of thepresent invention is not particularly restricted, and it can be producedby any method, which will provide phosphors according to the invention.A series of compositions of general formula of general formulaRE_(3-x)Al₂Al_(3-y)Si_(y)O_(12-y)N_(y):Ce_(x), wherein RE is a rareearth metal, selected from the group of yttrium, gadolinium, lutetium,terbium, scandium and lanthanum, and 0.002≦x≦0.2 and 0≦y≦3 can bemanufactured, which form a complete solid solution for the range of0.002≦x≦0.2 and 0≦y≦3.

A preferred process for producing a phosphor according to the inventionis referred to as the solid-state method. In this process, the phosphorprecursor materials are mixed in the solid state and are heated so thatthe precursors react and form a powder of the phosphor material.

The nitrides of the alkaline earth metals are mixed with amorphoussilicon nitride Si₃N₄, aluminum nitride AlN powder, the fluorides ofeuropium and/or cerium and a eventually flux in predetermined ratios.The mixture is placed into a high purity molybdenum crucible. Thecrucibles are loaded into a tube furnace and purged with flowingnitrogen/hydrogen for several hours. The furnace parameters are 10°C./min to 1450° C., followed by a 4 hour dwell at 1450° C. after whichthe furnace is slowly cooled to room temperature. The samples are onceagain finely ground before a second annealing step at 1400° C. isperformed. Luminous output may be improved through an additional thirdanneal at slightly lower 25 temperatures in flowing argon. After firing,the powders were characterized by powder X-ray diffraction (Cu,Kα-line), which showed that the desired phase had formed. An amber tored powder is obtained, which efficiently luminesces under UV and blueexcitation.

Preferably the europium-activated oxonitridoalumosilicate type phosphorsaccording to the invention may be coated with a thin, uniform protectivelayer of one or more compounds selected from the group formed by thefluorides and orthophosphates of the elements aluminum, scandium,yttrium, lanthanum gadolinium and lutetium, the oxides of aluminum,yttrium, silicon and lanthanum and the nitride of aluminum.

The protective layer thickness customarily ranges from 0.001 to 0.2 μmand, thus, is so thin that it can be penetrated by the radiation of theradiation source without substantial loss of energy. The coatings ofthese materials on the phosphor particles can be applied, for example,by deposition from the gas phase or a wet-coating process.

The phosphors according to the invention are, because of theiroxonitridoalumosilicate 15 structure, resistant to heat, light andmoisture.

When excited by radiation of the UVA or blue range of theelectromagnetic spectrum, these rare earth metal-activatedoxonitridoalumosilicate phosphors are found to give a broadbandemission, with peak wavelength in the amber to deep red range and a tailemission up to the infrared.

Preferably, europium(II) is used as activator. Table 1 gives thespectral data of some europium(II)-activated phosphors.

TABLE 1 Composition E_(m,max) QE₄₅₀ RQ₄₅₀ x y LE CaAlSi(N,O)₃:Eu(2%) 652nm 86.1 35.2 0.657 0.342 125.3 Ca_(0.95)Sr_(0.05)AlSi(N,O)₃:Eu(2%) 657nm 86.8 17.3 0.68 0.318 82 Ca_(0.95)Ba_(0.05)AlSi(N,O)₃:Eu(2%) 658 nm83.5 17.5 0.676 0.323 93.2 CaAl_(0.98)B_(0.02)Si(N,O)₃:Eu(2%) 650 nm81.6 19.4 0.667 0.332 108.1 CaAl_(0.95)B_(0.05)Si(N,O)₃:Eu(2%) 654 nm84.1 20.2 0.663 0.336 113 Ca_(0.95)Mg_(0.05)AlSi(N,O)₃:Eu(2%) 652 nm86.1 35.2 0.657 0.342 125.3 Ca_(0.9)Mg_(0.1)AlSi(N,O)₃:Eu(2%) 645 nm78.3 24.9 0.63 0.368 155.5 Ca_(0.5)Mg_(0.5)AlSi(N,O)₃:Eu(2%) 633 nm 70.836.6 0.578 0.418 222.4

Replacing some or all of the europium in an europium-activatedoxonitridoalumosilicate by cerium has the effect, that the ceriumproduces secondary emission that is concentrated in the yellow to amberregion of the visible spectrum, instead of a typical broadband secondaryemission from europium(II)-activated oxonitridoalumosilicate phosphorthat is generally centered in the deep red region of the visiblespectrum. Co-activators, such as ytterbium, praseodymium, samarium,terbium, thulium, dysprosium, holmium and erbium may also be used. Forcharge compensation monovalent cations like alkaline metals maybeincorporated into the host-lattice in amounts, which match or are belowthe amount of Ce(III) present.

Table 2 gives the spectral data of some cerium(III) activated compounds.

TABLE 2 Composition E_(m,max) QE₄₅₀ RQ₄₅₀ x y LE CaAlSiN₃:Ce(2%) 610 nm75.2 45.2 0.502 0.48 340 CaAlSiN₃:Ce,Eu(2%,0.1%) 631 nm 78.2 49.6 0.550.438 271.5 CaAlSiN₃:Ce,Na(2%,0.1%) 564 nm 76.9 54.5 0.471 0.505 383.3CaAlSiN₃:Ce(2%) 571 nm 82.5 43.4 0.47 0.51 400.6 CaAlSiN₃:Ce(2%) 569 nm81.8 39.9 0.472 0.508 399.5 CaAlSiN₃:Ce(4%) 583 nm 74.5 26.9 0.501 0.487366.4 CaAlSiN₃:Ce,Pr(2%,0.5%) 568 nm 74.5 36.5 0.477 0.503 390.1CaAlSiN₃:Ce(2%) w/SiO₂ coat 571 nm 69.9 30.1 0.476 0.505 391.5

The Illumination System

The invention also concerns an illumination system comprising aradiation source and a fluorescent material comprising at least one rareearth metal-activated oxonitridoalumosilicate of general formula(Ca_(1-x-y-z)Sr_(x)Ba_(y)Mg_(z))_(1-n)(Al_(1-a+b)B_(a))Si_(1-b)N_(3-b)O_(b):RE_(n),wherein 0≦x≦1, 0≦y≦1, 0≦z≦1, 0≦a≦1, 0<b≦1 and 0.002≦n≦0.2 and RE isselected from europium(II) and cerium(III).

Radiation sources include semiconductor optical radiation emitters andother devices that emit optical radiation in response to electricalexcitation. Semiconductor optical radiation emitters include lightemitting diode LED chips, light emitting polymers (LEPs), organic lightemitting devices (OLEDs), polymer light emitting devices (PLEDs), etc.

Moreover, light-emitting components such as those found in dischargelamps and fluorescent lamps, such as mercury low and high pressuredischarge lamps, sulfur discharge lamps, and discharge lamps based onmolecular radiators are also contemplated for use as radiation sourceswith the present inventive phosphor compositions.

In a preferred embodiment of the invention the radiation source is asolid-state light-emitting diode.

Any configuration of an illumination system which includes a solid stateLED and a rare earth metal-activated oxonitridoalumosilicate phosphorcomposition is contemplated in the present invention, preferably withaddition of other well-known phosphors, which can be combined to achievea specific colored amber, red or white light when irradiated by a LEDemitting primary UV or blue radiation as specified above.

A detailed construction of one embodiment of such illumination systemcomprising a radiation source and a fluorescent material shown in FIG. 1will now be described.

FIG. 1 shows a schematic view of a chip type light emitting diode with acoating comprising the fluorescent material. The device comprises chiptype light emitting diode (LED) 1 as a radiation source. Thelight-emitting diode die is positioned in a reflector cup lead frame 2.The die 1 is connected via a bond wire 7 to a first terminal 6, anddirectly to a second electric terminal 6. The recess of the reflectorcup is filled with a coating material that contains a fluorescentmaterial 4,5 according to the invention to form a coating layer that isembedded in the reflector cup. The phosphors are applied eitherseparately or in a mixture.

The coating material typically comprises a polymer 3 for encapsulatingthe phosphor or phosphor blend. In these embodiments, the phosphor orphosphor blend should exhibit high stability properties against theencapsulant. Preferably, the polymer is optically clear to preventsignificant light scattering. A variety of polymers are known in the LEDindustry for making LED lamps.

In one embodiment, the polymer is selected from the group consisting ofepoxy and silicone resins. Adding the phosphor mixture to a liquid thatis a polymer precursor can perform encapsulation. For example, thephosphor mixture can be a granular powder. Introducing phosphorparticles into polymer precursor liquid results in formation of a slurry(i.e. a suspension of particles). Upon polymerization, the phosphormixture is fixed rigidly in place by the encapsulation. In oneembodiment, both the fluorescent material and the LED dice areencapsulated in the polymer.

The transparent coating material may advantageously compriselight-diffusing particles, so-called diffusers. Examples of suchdiffusers are mineral fillers, in particular CaF₂, TiO₂, SiO₂, CaCO₃ orBaSO₄ or else organic pigments. These materials can be added in a simplemanner to the above-mentioned resins.

In operation, electrical power is supplied to the die to activate thedie. When activated, the die emits the primary light, e.g. blue light. Aportion of the emitted primary light is completely or partially absorbedby the fluorescent material in the coating layer. The fluorescentmaterial then emits secondary light, i.e., the converted light having alonger peak wavelength, primarily amber to red in a sufficientlybroadband (specifically with a significant proportion of deep red) inresponse to absorption of the primary light. The remaining unabsorbedportion of the emitted primary light is transmitted through thefluorescent layer, along with the secondary light. The encapsulationdirects the unabsorbed primary light and the secondary light in ageneral direction as output light. Thus, the output light is a compositelight that is composed of the primary light emitted from the die and thesecondary light emitted from the fluorescent layer.

The color temperature or color point of the output light of anillumination system according to the invention will vary depending uponthe spectral distributions and intensities of the secondary light incomparison to the primary light.

Firstly, the color temperature or color point of the primary light canbe varied by a suitable choice of the light emitting diode.

Secondly, the color temperature or color point of the secondary lightcan be varied by a suitable choice of the phosphor in the fluorescentmaterial, the particle size of the phosphor grains and the amount ofphosphor applied. Furthermore, these arrangements also advantageouslyafford the possibility of using phosphor blends in the fluorescentmaterial, as a result of which, advantageously, the desired hue can beset even more accurately.

The White Phosphor Converted Light Emitting Device

According to one aspect of the invention the output light of theillumination system may have a spectral distribution such that itappears to be “white” light. The most popular white LED's consist ofblue emitting LED chips that are coated with a phosphor that convertssome of the blue radiation to a complimentary color, e.g. a yellow toamber emission. Together the blue and yellow emissions produce whitelight. There are also white LED's which utilize a UV emitting chip andphosphors designed to convert the UV radiation to visible light.Typically, two or more phosphor emission bands are required.

Blue/Phosphor White LED (Dichromatic White Light Phosphor ConvertedLight Emitting Device Using Blue Emitting Light Emitting Diode)

In a first embodiment, a white-light emitting illumination systemaccording to the invention can advantageously be produced by choosingthe fluorescent material such that a blue radiation emitted by a bluelight emitting diode is converted into complementary wavelength ranges,to form dichromatic white light. In this case amber to red light isproduced by means of the fluorescent materials that comprise a rareearth metal-activated oxonitridoalumosilicate phosphor. Also a secondfluorescent material can be used, in addition, in order to improve thecolor rendition of this illumination system.

Particularly good results are achieved with a blue LED whose emissionmaximum lies at 400 to 490 nm. An optimum has been found to lie at 440nm, taking particular account of the excitation spectrum of the rareearth metal-activated oxonitridoalumosilicate.

When compared with the spectral distribution of the white output lightgenerated by the prior art LED the apparent difference in the spectraldistribution is the shift of the peak wavelength which is in the redregion of the visible spectrum. Thus, the white output light generatedby the illumination system has a significant additional amount of redcolor, as compared to the output light generated by the prior art LED.

(Polychromatic White Light Phosphor Converted Light Emitting DeviceUsing Blue Emitting Light Emitting Diode).

In a second embodiment, a white-light emitting illumination systemaccording to the invention can advantageously be produced by choosingthe fluorescent material such that a blue radiation emitted by the bluelight emitting diode is converted into complementary wavelength ranges,to form polychromatic white light. In this case, amber to red light isproduced by means of the fluorescent materials that comprise a blend ofphosphors including rare earth metal-activated oxonitridoalumosilicatephosphor and a second phosphor.

Yielding white light emission with even high color rendering is possibleby using additional red and green broad band emitter phosphors coveringthe whole spectral range together with a blue-emitting LED and a amberto red emitting rare earth metal activated oxonitridoalumosilicatephosphor.

Useful second phosphors and their optical properties are summarized inthe following table 3.

TABLE 3 Composition λ_(max) x Y (Ba_(1−x)Sr_(x))₂SiO₄:Eu 523 nm 0.2720.640 SrGa₂S₄:Eu 535 nm 0.270 0.686 SrSi₂N₂O₂:Eu 541 nm 0.356 0.606Y₃Al₅O₁₂:Ce 550 nm 0.447 0.535 SrS:Eu 610 nm 0.627 0.372(Sr_(1−x−y)Ca_(x)Ba_(y))₂Si₅N₈:Eu 615 nm 0.615 0.384(Sr_(1−x−y)Ca_(x)Ba_(y))₂Si_(5−a)Al_(a)N_(8−a)O_(a):Eu 615-650 nm   0.633 0.366 CaS:Eu 655 nm 0.700 0.303 (Sr_(1−x)Ca_(x))S:Eu 610-655 nm   

The fluorescent materials may be a blend of two phosphors, an amber tored rare earth metal-activated oxonitridoalumosilicate phosphor and ared phosphor selected from the group (Ca_(1-x)Sr_(x))S:Eu, wherein 0≦x≦1and (Sr_(1-x-y)Ba_(x)Ca_(y))₂Si_(5-a)Al_(a)N_(8-a)O_(a):Eu wherein0≦a≦5, 0≦x≦1 and 0≦y≦1.

The fluorescent materials may be a blend of two phosphors, e.g. an amberto red rare earth metal-activated oxonitridoalumosilicate phosphor and agreen phosphor selected from the group comprising(Ba_(1-x)Sr_(x))₂SiO₄:Eu, wherein 0≦x≦1, SrGa₂S₄:Eu and SrSi₂N₂O₂:Eu.

The hue (color point in the CIE chromaticity diagram) of the white lightthereby produced can in this case be varied by a suitable choice of thephosphors in respect of mixture and concentration.

UV/Phosphor White LED

(Dichromatic white phosphor converted light emitting device usingUV-emitting light) In a further embodiment, a white-light emittingillumination system according to the invention can advantageously beproduced by choosing the fluorescent material such that a UV radiationemitted by the UV light emitting diode is converted into complementarywavelength ranges, to form dichromatic white light. In this case, theamber and blue light is produced by means of the fluorescent materials.Amber light is produced by means of the fluorescent materials thatcomprise a rare earth metal-activated oxonitridoalumosilicate phosphor.Blue light is produced by means of the fluorescent materials thatcomprise a blue phosphor selected from the group comprisingBaMgAl_(1o)O₁₇:Eu, Ba₅SiO₄(Cl,Br)₆:Eu, CaLn₂S₄:Ce and(Sr,Ba,Ca)₅(PO₄)₃Cl:Eu. Particularly good results are achieved inconjunction with a UVA light emitting diode, whose emission maximum liesat 200 to 400 nm. An optimum has been found to lie at 365 nm, takingparticular account of the excitation spectrum of the rare earth metalactivated oxonitridoalumosilicate.

Polychromatic White Phosphor Converted Light Emitting Device Using UVEmitting-LED

In a further embodiment, a white-light emitting illumination systemaccording to the invention can advantageously be produced by choosingthe fluorescent material such that UV radiation emitted by a UV emittingdiode is converted into complementary wavelength ranges, to formpolychromatic white light e.g. by additive color triads, for exampleblue, green and red.

In this case, the amber to red, the green and blue light is produced bymeans of the fluorescent materials.

Also a second red fluorescent material can be used, in addition, inorder to improve the color rendition of this illumination system.

Yielding white light emission with even higher color rendering ispossible by using blue and green broad band emitter phosphors coveringthe whole spectral range together with a UV emitting LED and an amber tored emitting rare earth metal-activated oxonitridoalumosilicatephosphor.

The hue (color point in the CIE chromaticity diagram) of the white lightthereby produced can in this case be varied by a suitable choice of thephosphors in respect of mixture and concentration.

The Amber to Red Phosphor Converted Light Emitting Device

According to one preferred aspect of the invention an illuminationsystem that emits output light having a spectral distribution such thatit appears to be amber or red light is provided.

Fluorescent material comprising rare earth metal-activatedoxonitridoalumosilicate according to the invention as phosphor isparticularly well suited as an amber or red component for stimulation bya primary UVA or blue radiation source such as, for example, anUVA-emitting LED or blue-emitting LED.

It is possible thereby to implement an illumination system emitting inthe amber to red regions of the electromagnetic spectrum.

In a first embodiment, a amber to red-light emitting illumination systemaccording to the invention can advantageously be produced by choosingthe fluorescent material such that a blue radiation emitted by the bluelight emitting diode is converted to amber or red light.

In this case, colored light is produced by means of the fluorescentmaterials that comprise a rare earth metal-activatedoxonitridoalumosilicate phosphor according to the invention.

Particularly good results are achieved with a blue LED the emissionmaximum of which lies at 400 to 495 nm. An optimum has been found to lieat 445 to 465 nm, taking particular account of the excitation spectrumof the oxonitridoalumosilicate phosphor.

The color output of the LED-phosphor system is very sensitive to thethickness of the phosphor layer. If the phosphor layer is thick andcomprises an excess of an amber or red emitting rare earthmetal-activated oxonitridoalumosilicate phosphor, then a lesser amountof the blue LED light will penetrate through the thick phosphor layer.The combined LED-phosphor system will then appear amber to red, becauseit is dominated by the amber to red secondary light of the phosphor.Therefore, the thickness of the phosphor layer is a critical variableaffecting the color output of the system.

The hue (color point in the CIE chromaticity diagram) of the amber tored light thereby produced can in this case be varied by a suitablechoice of the phosphor in respect of mixture and concentration.

The color points of the illumination systems according to thisembodiment are in the amber to red spectral range in the chromaticitydiagram of the Commission internationale de l'eclairage (“CIE”).

A red-light emitting illumination system according to the invention canparticularly preferably be realized by admixing an excess of theinorganic fluorescent material(Ca_(0.95)Sr_(0.05))_(0.98)Al_(1.04)Si_(0.96)N_(2.96)O_(0.04):Eu_(0.02)with a silicon resin used to produce the luminescence conversionencapsulation or layer. Most of a blue radiation emitted by a 462 nmInGaN light emitting diode is shifted by the inorganic fluorescentmaterial(Ca_(0.95)Sr_(0.05))_(0.98)Al_(1.04)Si_(0.96)N_(2.96)O_(0.04):Eu_(0.02)into the red spectral region. A human observer perceives the combinationof the remaining blue primary light and the excess secondary light ofthe phosphor as red light.

Amber to red LEDs with a color point on the line connecting x=0.58,y=0.42 and x=0.69, y=0.31 can be realized by this single phosphordichromatic concept, if the appropriate member of the compositionaccording to the formula(Ca_(1-x-y-z)Sr_(x)Ba_(y)Mg_(z))_(1-n)(Al_(1-a+b)B_(a))Si_(1-b)N_(3-b)O_(b):RE_(n)is chosen, as shown in FIG. 3.

In a second embodiment, colored-light emitting illumination systemaccording to the invention can advantageously be produced by choosingthe fluorescent material such that a blue radiation emitted by the bluelight emitting diode is converted into complementary wavelength ranges,to form polychromatic amber or red light. In this case, colored light isproduced by means of the fluorescent materials that comprise a blend ofphosphors including rare earth metal-activated oxonitridoalumosilicatephosphor and a second phosphor.

Useful second phosphors and their optical properties are summarized inthe following table 4:

TABLE 4 Composition λ_(max) x Y (Ba_(1−x)Sr_(x))₂SiO₄:Eu 523 nm 0.2720.640 SrGa₂S₄:Eu 535 nm 0.270 0.686 SrSi₂N₂O₂:Eu 541 nm 0.356 0.606Y₃Al₅O₁₂:Ce 550 nm 0.447 0.535 SrS:Eu 610 nm 0.627 0.372(Sr_(1−x−y)Ca_(x)Ba_(y))₂Si₅N₈:Eu 615 nm 0.615 0.384(Sr_(1−x−y)Ca_(x)Ba_(y))₂Si_(5−a)Al_(a)N_(8−a)O_(a):Eu 615-650 nm   0.633 0.366 CaS:Eu 655 nm 0.700 0.303 (Sr_(1−x)Ca_(x))S:Eu 610-655 nm   

In a further aspect of the invention, an amber or red-light emittingillumination system according to the invention can advantageously beproduced by choosing the fluorescent material such that a UV radiationemitted by the UV emitting diode is converted entirely intomonochromatic amber to red light. In this case, the amber to red lightis produced by means of the fluorescent materials.

In a first embodiment, an amber to red-light emitting illuminationsystem according to the invention can advantageously be produced bychoosing the fluorescent material such that a blue radiation emitted bythe UV light emitting diode is converted into complementary wavelengthranges, to form dichromatic colored, especially amber or red, light.

In this case, colored light is produced by means of the fluorescentmaterials that comprise a rare earth metal-activatedoxonitridoalumosilicate phosphor.

Particularly good results are achieved with a UV-LED whose emissionmaximum lies at near UV from 370 to 430 nm

The color output of the illumination system comprising an UV-LED asradiation source is not very sensitive to the thickness of the phosphorlayer. Therefore, the thickness of the phosphor layer is a not criticalvariable affecting the color output of the system and may be reduced.

The hue (color point in the CIE chromaticity diagram) of the amber orred light thereby produced can in this case be varied by a suitablechoice of the phosphor in respect of mixture and concentration.

The color points of the illumination systems according to thisembodiment are in the amber to red spectral range in the chromaticitydiagram of the Commission internationale de l'eclairage (“CIE”).

In many of the examples described above, multiple phosphors are includedin a single illumination system. When some phosphors are mixed together,interaction between the mixed phosphors may adversely affect theefficiency and spectrum of the device. For example, amber-to-redemitting phosphors may absorb much of the light emitted byyellow-to-green emitting phosphor. Accordingly, depending on thephosphors in the combination, the phosphors may be formed as separate,discrete layers, or mixed and formed as a single layer. The preferredphosphor arrangement may depend on the excitation and emission spectraof the phosphors and on the application. Also, the phosphor arrangementmay be chosen to maximize a particular property of the combinedspectrum, such as, for example, the color rendering index, given as CRIor Ra, the color gamut, or the luminous equivalent. The luminousequivalent is the highest efficiency possible for a given spectrum andis expressed in lumens/W.

In a variation of the device illustrated in FIG. 1, multiple phosphorsmay be formed as discrete layers disposed next to each other in areflector cup. For example, a yellow-to-green emitting phosphor may bemixed with a resin, silicone, or other transparent material and disposedon one side of the reflector cup, while any other phosphors, includingan amber-to-red emitting phosphor, are mixed separately with a resin,silicone, or other transparent material and disposed on the other sideof reflector cup, such that the slurry containing the yellow-to-greenemitting phosphor does not appreciably mix with the slurry containingthe amber-to-red emitting phosphor. The viscosity of the transparentmaterial forming the slurries may be selected to avoid mixing the twoslurries. Since the yellow-to-green emitting phosphor and any otherphosphors are adjacent to each other, rather than mixed in the sameslurry, light emitted by yellow-to-green emitting phosphor is lesslikely to be absorbed by any amber-to-red emitting phosphors in theother slurry.

FIG. 5 shows an alternative embodiment of an illumination system withmultiple phosphors, where different phosphors are deposited over the LEDas discrete layers. Phosphor layer 40, including any amber-to-redemitting phosphors, is deposited closest to LED 10. Yellow-to-greenemitting phosphor 50 is then deposited over phosphor layer 40. Phosphorlayers 40 and 50 may be separated by an optional transparent layer 60.Though phosphor layers 40 and 50 are shown covering the sides of LED 10,for design considerations or depending on the technique used to form thephosphor layer, one or both of phosphor layers 40 and 50 may not coverthe entire top surface of LED 10, and/or may not extend over the sidesof LED 10.

Phosphor layers 40 and 50 may be formed as ceramics as described in U.S.Published Application 2005/0269582; deposited as slurries in a resin orother transparent material; deposited as thin films by, for example,electron beam evaporation, thermal evaporation, rf-sputtering, chemicalvapor deposition, or atomic layer epitaxy; or deposited as conformallayers over LED 10 by, for example, screen printing, stenciling asdescribed in U.S. Pat. No. 6,650,044, or by electrophoretic depositionas described in U.S. Pat. No. 6,576,488. Thin films are described inmore detail in U.S. Pat. No. 6,696,703. Each of U.S. PublishedApplication 2005/0269582, U.S. Pat. No. 6,696,703, U.S. Pat. No.6,650,044 and U.S. Pat. No. 6,576,488 are incorporated herein byreference. In contrast to a thin film, which typically behaves as asingle, large phosphor particle, the phosphor in a conformal layergenerally behaves as multiple phosphor particles. In addition a thinfilm typically contains no materials other than phosphor. A conformallayer often includes materials other than phosphor, such as, forexample, silica. Phosphor layers 40 and 50 need not be formed by thesame technique. For example, a first phosphor may be formed as a ceramiclayer or a conformal layer disposed on the LED, which is then coatedwith a slurry including a second phosphor.

One or more dichroic filters may be included in the device. For example,a dichroic filter designed to transmit light emitted by LED 10 but toreflect light emitted by phosphors 40 and 50 may be included between LED10 and phosphor layer 40. Layer 60 between yellow-to-green emittingphosphor 50 and amber-to-red emitting phosphor 40 may be a dichroicfilter designed to transmit light emitted by amber-to-red emittingphosphor 40 and LED 10, and reflect light emitted by yellow-to-greenemitting phosphor 50. Dichroic filters may reduce the amount ofradiation back-scattered by phosphor layers 40 and 50 into LED 10, whereit can be absorbed.

As an alternative to the device of FIG. 5, the yellow-to-green emittingphosphor and other phosphors may be deposited on the LED in a pluralityof small regions, which may form a pattern, such as a checkerboardpattern. Patterns of different phosphor layers may be formed bydepositing a first layer of phosphor by electrophoretic deposition,patterning that layer using conventional lithography and etchingtechniques, then depositing a second phosphor layer by electrophoreticdeposition. Alternatively, patterns of phosphor layers may be depositedby screen printing or ink jet printing. In some embodiments, a patternof phosphor layers may be formed by pipetting the individual phosphormixes into wells in a clear plastic microplate used for microbiology.The phosphor-filled microplate is then placed on the LED.Phosphor-filled microplates may be formed separately from the LED.Alternatively, a plurality of small regions of one phosphor may beformed on the surface of the LED, then a layer of a second phosphor maybe deposited over the plurality of regions of the first phosphor.

Devices including multiple phosphors formed as discrete layers isdescribed in more detail in U.S. Published Application 2005/0184638,which is incorporated herein by reference.

EXAMPLE

In one example, a rare earth metal-activated oxonitridoalumosilicate ofgeneral formula(Ca_(1-y-z)Sr_(x)Ba_(y)Mg_(z))_(1-n)(Al_(1-a+b)B_(a))Si_(1-b)N_(3-b)O_(b):RE_(n),wherein 0≦x≦1, 0≦y≦1, 0≦z≦1, 0≦a≦1, 0<b≦1 and 0.002≦n≦0.2 and RE isselected from europium(II) and cerium(III) is configured to emit redlight and is combined with an LED configured to emit blue light, forexample at a peak wavelength of about 450 nm, and a Y₃Al₅O₁₂:Ce³⁺phosphor configured to emit yellow light. Unconverted blue light fromthe light emitting diode mixes with light emitted by the two phosphorssuch that the composite light appears white. Such a white light deviceoffers several benefits.

First, red-emitting(Ca_(1-x-y-z)Sr_(x)Ba_(y)Mg_(z))_(1-n)(Al_(1-a+b)B_(a))Si_(1-b)N_(3-b)O_(b):RE_(n)does not degrade when used in high humidity environments, even without acoating against moisture. Other red-emitting phosphors exhibit adecrease in quantum efficiency when left uncoated and used in a highhumidity environment.

Second, red-emitting(Ca_(1-x-y-z)Sr_(x)Ba_(y)Mg_(z))_(1-n)(Al_(1-a+b)B_(a))Si_(1-b)N_(3-b)O_(b):RE_(n)is stable when driven at high current or operated at high temperature.As the drive current increases in devices including other red-emittingphosphors such as CaS, the height of the blue peak in the spectrumrelative to the red peak increases, indicating that the phosphor absorbsfewer photons as the drive current increases, and therefore becomes lessefficient. In contrast, in a white light device including a red-emitting(Ca_(1-x-y-z)Sr_(x)Ba_(y)Mg_(z))_(1-n)(Al_(1-a+b)B_(a))Si_(1-b)N_(3-b)O_(b):RE_(n),the relative sizes of the red and blue peaks do not significantly changeas the drive current increased to 4 A in a 1 mm² device, when at atemperature less than 200° C.

Third, red-emitting(Ca_(1-x-y-z)Sr_(x)Ba_(y)Mg_(z))_(1-n)(Al_(1-a+b)B_(a))Si_(1-b)N_(3-b)O_(b):RE_(n)may be configured to emit deeper red light than other red-emittingphosphors, which may improve the color rendering of a device including ablue-emitting LED, red-emitting(Ca_(1-x-y-z)Sr_(x)Ba_(y)Mg_(z))_(1-n)(Al_(1-a+b)B_(a))Si_(1-b)N_(3-b)O_(b):RE_(n)and a yellow-emitting phosphor such as Y₃Al₅O₁₂:Ce³⁺. For example, adevice including a blue-emitting LED, red-emitting(Ca_(1-x-y-z)Sr_(x)Ba_(y)Mg_(z))_(1-n)(Al_(1-a+b)B_(a))Si_(1-b)N_(3-b)O_(b):RE_(n)and Y₃Al₅O₁₂:Ce³⁺ emits light having a color rendering index Ra greaterthan 85 and a value R9, which is an indication of the amount of deep redemission, greater than 40 at a color temperature CCT between 3000 and3500K. Higher Ra and R9 values indicate better color rendering. Incontrast, a Warm White LED available from Nichia Corporation emits lighthaving a color rendering index Ra of only 74 and an R9 value of zero ata color temperature less than 2800 K, indicating poor color renderingand very little deep red emission. Red-emitting(Ca_(1-x-y-z)Sr_(x)Ba_(y)Mg_(z))_(1-n)(Al_(1-a+b)B_(a))Si_(1-b)N_(3-b)O_(b):RE_(n)phosphors may thus be used to make devices emitting composite lighthaving a Ra of at least 80 and an R9 of at least 20.

Fourth, red-emitting(Ca_(1-x-y-z)Sr_(x)Ba_(y)Mg_(z))_(1-n)(Al_(1-a+b)B_(a))Si_(1-b)N_(3-b)O_(b):RE_(n)may be efficiently combined with other phosphors in white-light emittingdevices. For example, a device combining red-emitting(Ca_(1-x-y-z)Sr_(x)Ba_(y)Mg_(z))_(1-n)(Al_(1-a+b)B_(a))Si_(1-b)N_(3-b)O_(b):RE_(n)with Y₃Al₅O₁₂:Ce³⁺ and a blue-emitting LED emits a combined spectrumwith a higher luminous equivalent than is predicted by a simplesimulation assuming a superposition of the individual spectra of the LEDand the two phosphors. Accordingly, the interactions betweenred-emitting(Ca_(1-x-y-z)Sr_(x)Ba_(y)Mg_(z))_(1-n)(Al_(1-a+b)B_(a))Si_(1-b)N_(3-b)O_(b):RE_(n)and Y₃Al₅O₁₂:Ce³⁺ do not reduce the expected efficiency of the device,and may enhance it.

In some embodiments, a(Ca_(1-x-y-z)Sr_(x)Ba_(y)Mg_(z))_(1-n)(Al_(1-a+b)B_(a))Si_(1-b)N_(3-b)O_(b):RE_(n)phosphor may be synthesized as described below. The use of fluxes, i.e.additives that form liquid phases at elevated temperatures duringsynthesis, is a method in solid-state synthesis for lowering synthesistemperatures and improving particle morphology and crystallinity. Ingeneral the choice of an appropriate flux is limited to compounds withmelting points close to the synthesis temperature, which eitherevaporate from the synthesis mixture or can be incorporated into thecrystal lattice of the target phosphor. In the case of a CaAlSiN₃:Euphosphor, salts like CaF₂ or SrF₂ may be used, since the melting pointsof CaF₂ and SrF₂ are 1402° C. and 1463° C., respectively and Ca, Sr andF are readily incorporated into the CaAlSiN₃ crystal structure.Potential drawbacks to using fluxes include possible changes in thecolor point of the spectrum emitted by the phosphor, as well as theformation of defects due to fluoride uptake of the host lattice.

Surprisingly, the inventors have discovered that in addition to CaF₂ orSrF₂, NH₄Cl and NaCl, which melt at 300° C. and 800° C., respectively,also work well as fluxes. As both compounds have high vapor pressures atthe synthesis temperature of CaAlSiN₃, they are not incorporated intothe crystal lattice of the target compound, and thus may avoid thepotential drawbacks of CaF₂ and SrF₂.

For example, (Ca_(0.95),Sr_(0.05))AlSiN₃:Eu (2%) was synthesised asfollows: all actions are carried out in dry inert gas atmosphere. 4.099g AlN (Tokuyama, Tokyo, Japan), 4.942 g Ca₃N₂ (ESPI, Ashland, Oreg.USA), 0.352 g Eu₂O₃ (Alfa Aesar, Karlsruhe, Germany), 4.732 g Si₃N₄ (UBEEurope GmbH, Diisseldorf, Germany) and 0.448 g SrH₂ (Alfa Aesar,Karlsruhe, Germany) are weighed into an agate milling jar. 35 mLTetrahydrofuran (Aldrich, Taufkirchen, Germany) and 20 agate millingballs (diameter 5 mm) are added. The powders are milled in a planetaryball mill at 200 rpm for 20 minutes. The dried mixture is transferredinto a SiC crucible with lid and fired at 1300° C. for 4 hours. Thepre-reacted powder is collected, 1% w/w flux is added and the ballmilling is repeated. The mixture with flux is transferred back into theSiC crucible with lid and fired for a second time at 1600° C. for 4hours. The final reaction product is of light red color and stable inambient air. It is deagglomerated using the planetary ball mill and thenwashed with acetic acid, water and ethanol.

Using this recipe five powders were obtained using different fluxes: A:NH₄Cl; B: CaF₂; C: SrF₂; D: NaCl; and E: no flux. Table 5 illustratesthe spectral properties obtained for the 5 examples. In table 5, QE₄₅₀refers to the quantum efficiency at 450 nm excitation, RQ₄₅₀ refers tothe reflection at 450 nm, x and y refer to coordinates on the 1931 CIEchromaticity chart, and LE refers to the lumen equivalent. Particularlyfavorable properties (such as low RQ₄₅₀, high QE₄₅₀, high x, and low ychromaticity coordinates) are observed for samples A, D and E. The goodabsorption and deep red color point of example A are favorable for someof the examples and embodiments described above.

TABLE 5 Spectral properties of (Ca_(0.95),Sr_(0.05))AlSiN₃:Eu (2%)powders prepared using different fluxes. Sample QE₄₅₀ (%) RQ₄₅₀ (%) x yLE (lm/W) A 86.4 24.9 0.663 0.336 112.5 B 88.3 30.6 0.648 0.350 133.1 C85.5 30.1 0.649 0.349 130.5 D 84.2 29.3 0.653 0.346 128.7 E 87.1 27.00.657 0.342 121.1

The above synthesis technique for CaAlSiN₃:Eu may avoid problems withother techniques caused by the loss of Ca from the compound at elevatedtemperatures. The loss of Ca may result in impaired optical propertiesof the resulting material, due to defects in the host lattice. GaseousCa may also corrode furnace equipment. The above synthesis techniqueavoids formation of gaseous Ca without undesirably lowering firingtemperatures and without requiring synthesis in pressurised furnaces,both of which may have drawbacks. For example, lower firing temperaturesmay result in less crystalline finer grained particles, usually havinglower absorption at the excitation wavelength and lower light yields inthe final devices; and the use of pressurised furnace equipmentgenerally increases the difficulty of production as well as productionand energy costs.

Having described the invention in detail, those skilled in the art willappreciate that, given the present disclosure, modifications may be madeto the invention without departing from the spirit of the inventiveconcept described herein. Therefore, it is not intended that the scopeof the invention be limited to the specific embodiments illustrated anddescribed.

1-17. (canceled)
 18. A method of forming a phosphor comprising a rareearth metal-activated oxonitridoalumosilicate of general formula (Ca₁_(—) _(x) _(—) _(y) _(—)_(z)Sr_(x)Ba_(y)Mg_(z))_(1-n)(Al_(1-a+b)Ba)Si_(1-b)N_(3-b)O_(b):RE_(n),wherein 0≦x≦1, 0≦y≦1, 0≦z≦1, 0≦a≦1, 0<b≦1 and 0.002≦n≦0.2 and RE isselected from europium(II) and cerium(III), the method comprising:milling at least one precursor material with a flux; and firing the atleast one precursor material and the flux at a first temperature;wherein: the first temperature is greater than room temperature; and theflux forms a liquid phase at the first temperature.
 19. The method ofclaim 8 wherein the flux comprises NH₄Cl.
 20. The method of claim 8wherein the flux is selected from a group including CaF₂, SrF₂, andNaCl.